Portion of mask frame



FIG. 1 is a perspective view of a full-face mask according to an embodiment of my design;

FIG. 2 is a front view thereof;

FIG. 3 is a rear view thereof; and,

FIG. 4 is a right side view thereof with the left side view thereof being a mirror image thereof.

Where included, broken lines showing environmental structure are for illustrative purposes only and form no part of the claimed design. 

The ornamental design for a portion of mask frame, as shown and described. 